FIG. 1 illustrates a relay node (RN) 120 and user equipments (UE) 131 and 132, which exist within one base station (eNodeB; eNB) 110 in a wireless communication system 100. The relay node 120 may transfer data received from the base station 110 to the user equipment 132 therein and transfer the data received from the user equipment 132 therein to the base station 110. Also, the relay node 120 may extend a high data rate region, enhance communication quality at a cell edge, and support communication inside a building or a zone beyond base station service coverage. In FIG. 1, a user equipment (hereinafter, referred to as macro-user equipment (Macro-UE)) such as the user equipment 131, which directly receives a service from the base station, and a user equipment (hereinafter, referred to as relay-user equipment (Relay-UE)) such as the user equipment 132, which receives a service from the relay node 120, are shown.
A wireless link between the base station and the relay node will be referred to as a backhaul link, a link from the base station to the relay node will be referred to as a backhaul downlink, and a link from the relay node to the base station will be referred to as a backhaul uplink. A link from the relay node to the user equipment will be referred to as an access downlink, and a link from the user equipment to the relay node will be referred to as an access uplink.
In the mean time, a multiple input multiple output (MIMO) system means a system that enhances transmission and reception efficiency of data by using multiple transmitting antennas and multiple receiving antennas. The MIMO technology may be divided into a spatial multiplexing scheme and a spatial diversity scheme. As the spatial diversity scheme may enhance transmission reliability through diversity gain or enlarge cell radius, it is suitable for data transmission for a user equipment which moves at high rate. The spatial multiplexing scheme may increase a data transmission rate without increasing a system bandwidth by simultaneously transmitting different data at the same time.
A single-cell MIMO operation may be divided into a single user-MIMO (SU-MIMO) operation and a multi user-MIMO (MU-MIMO) operation. According to the single user-MIMO operation, one user equipment receives a downlink signal on a specific physical resource block (PRB) in one cell. According to the multi user-MIMO operation, two or more user equipments (or one or more user equipments and one or more relay nodes) receive a downlink signal on a specific PRB in one cell.
In the MIMO system, each transmitting antenna has an independent data channel. The transmitting antenna may mean a virtual antenna or a physical antenna. The receiver receives data transmitted from each transmitting antenna by estimating a channel for each transmitting antenna. Channel estimation means a procedure of recovering a received signal by compensating for signal distortion caused by fading. In this case, fading means that signal strength is rapidly changed due to multi path-time delay in a wireless communication system environment. For channel estimation, a reference signal known by both a transmitter and a receiver is required. Also, the reference signal may be referred to as a pilot signal in accordance with the application standard.
A downlink reference signal is a pilot signal for coherent demodulation such as a physical downlink shared channel (PDSCH), a physical control format indicator channel (PCFICH), a physical hybrid indicator channel (PHICH), and a physical downlink control channel (PDCCH). Examples of the downlink reference signal include a common reference signal (CRS) shared by all the user equipments within a cell and a dedicated reference signal (DRS) for a specific user equipment only. The common reference signal may be referred to as a cell-specific reference signal. Also, the dedicated reference signal may be referred to as a user equipment-specific (UE-specific) reference signal or a demodulation reference signal (DMRS).